Redundant Array of Inexpensive Disks, or sometimes alternatively Redundant Array of Independent Disks (RAID) is a technology that employs the simultaneous use of two or more storage mediums (e.g., hard disk drives, flash or solid state memories, etc.) to achieve greater levels of performance, reliability, and/or larger data volume sizes. Frequently, the term “RAID” is often used as an umbrella term for data storage schemes that can divide and replicate data among multiple storage mediums, regardless of price or the storage medium's independent nature.
Generally, a RAID array distributes data across several storage mediums, but the array is seen by the computer user and operating system levels as one single disk. Often a RAID array includes some form of data redundancy, such that, the failure of one (and sometimes more) disks or storage mediums in the array will not result in loss of data. In such an embodiment, a failed disk may be replaced by a new one, and the data on the failed disk can be reconstructed from the remaining disks.
There are various combinations of RAID approaches which strike different balances between the competing goals of performance, reliability, and/or larger data volume sizes. RAID levels 0, 1, and 5 are the most commonly found, and cover most requirements.
RAID 0 (a.k.a., striped disks) is generally used to increase data volume size and distributes data across several disks in a way that gives improved speed and allows a user access to the full capacity of the disks, but provides no redundancy. In fact, if any one of the disks fails, the data storage on the entire array is often lost. RAID 1 (a.k.a., mirrored disks) is generally used to optimize redundancy at the cost of capacity and often uses two or more disks that each store the same data so that data is not lost as long as one disk survives. Total capacity of the array is just the capacity of a single disk. RAID 5 (a.k.a., striped disks with parity) generally strikes a balance between capacity and redundancy at the price of speed, and combines three or more disks in a way that protects data against loss of any one disk; the storage capacity of the array is reduced by one disk. Other RAID approaches (e.g., RAID 6, RAID 1+0, etc.) may be used in various embodiments.